Tension test is the most common test used today to measure the mechanical properties of different materials. It is widely used to obtain basic design information about the strength of materials. It is an acceptance test for the determination of specifications of materials.
Mechanical testing plays an important role in the evaluation of the fundamental properties of materials. It is used in developing new materials and in optimizing the quality of materials for their use in construction and design. These materials need to be tested in for their strength, rigidity, toughness, etc., when subjected to a load. The study of these parameters is done by plotting a stress-strain curve, using the results obtained.
The important parameters that describe the stress strain curve are:
Ultimate tensile strength (UTS)
Reduction in area
To obtain the stress strain diagram, determine the tensile properties and get the information about the mechanical behavior and the engineering performance.
The testing system consists of tensile testing machine, a power supply, a load cell and x-y recorder. The most common machine used today is the UNIVERSAL TESTING MACHINE. This machine has two crossheads: one adjusted for the length of specimen and the other for applying tension to the test specimen.
There are two types:
Hydraulic powered machine
Electromagnetically powered machine
The machine must have proper capabilities for the proper testing of the specimen. There are four main important parameters: Force capacity, speed, precision and accuracy
The preparation of test specimens depends on the purpose of testing. They are machined in the required orientation and according to the standards. A tensile specimen usually is a standardized sample cross section. It has two shoulders and a gage (narrow section) in between. The shoulders are large so that they can be gripped easily and the gage section has lesser cross section so that the deformation and failure can occur in this region. This ensures that the failure occurs at a section which is not affected by the gripping device. The elongation is measured between the gage length markings during the test.
Tensile test specimen as per ASTM standards
www.reasearchgate.net, Synthesis and Characterization of SIC Reinforced HE- 30 Al Alloy Particulate MMC’s
Full text (pdf) available from – enggjournals.com,Aug-10, 2016License: CC BY 4.0The shoulders of the test specimen can be manufactured in various ways to obtain different kinds of grips. This is done to make sure that the alignment is accurate, manufacturing cost isn’t too high and gripping are easy. A pinned grip and threaded shoulders and grips assure good alignment
1. Tensile strength (UTS) =; ?UTS=Pmax/ Ao
2. Percentage elongation =; z = %? = (lf –lo)/lo*100
3. Reduction in area =; q = %RA = (Ao – Af)/ Ao*100
4. Poisson’s ratio => v = (?d/ do)/(?l/lo)
Pmax – Maximum load Ao – Initial cross sectional area
? – Elongation Af – Final cross sectional area
lo – Initial length lf – Final length
?d -Change in width do – Initial width
?l – Change in length
The ASTM D638 is one of the most common tensile testing protocols. It measures the plastics tensile properties including ultimate tensile strength, yield strength, elongation and Poisson’s ratio.
Before the test:
Put gage marks on the given specimen.
Measure initial gage diameter and length
Select a load scale to deform and fracture the given specimen. The tensile strength of the material type used has to be known (approximately).
During the test:
Record the maximum load applied
Conduct the test till fracture occurs.
After the test:
Measure the final gage diameter and length. The diameter should be measured at the neck.
From the obtained data, stress strain curve is plotted and the following is calculated
1. Yield stress 2. Young’s modulus
3. UTS 4. Fracture stress
5. Yielding strain 6. Strain at onset of neck
7. Fracture strain 8. Percentage elongation
9. Percentage reduction in area 10. Resilience
12. Poisson’s ratio assuming the volume is constant during the elastic deformation
The results of tension test may not totally represent the strength and ductility properties of the entire end product or it’s in service behavior in different environments.
By using such tests, stress strain data can be obtained for different materials. This data is can be used for other experiments and extended research.